Outdoorsy people have long enjoyed hiking on remote trails or rafting on rivers. But aerial views have been harder to come by, until the recent popularization of zip lines. Although varieties of zip lines have existed for hundreds of years, they have become widespread in recent years. It’s a popular outdoor activity, but how does it impact trees?

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Zip lines originated 2,000 years ago in mountainous areas of China, India and Japan, where people first used them to transport supplies from one place to another and to traverse dangerous areas. Later, European mountaineers navigated between high points with zip lines. In the 1970s, wildlife biologists built zip lines in the Costa Rican rainforests to cause minimal environmental disturbance during their studies. These Costa Rican ziplines soon caught the attention of tourists, and entrepreneurs latched onto the idea, opening recreational zip lines in Costa Rica and then other parts of the world.

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The United States’ first recreational zipline, the Haleakala Skyline Tour in Maui, launched in 2002. Fewer than 20 years later, commercial zip lines operate in at least 72 countries on six continents, with more than 400 zip courses in the U.S. alone.

Obviously, zip lining is popular with people. But what effect does it have on trees and forests? Inhabitat spoke with four zip-line experts to get their views on the subject.

person zip lining and another person watching and waiting to zip line

Designing a zip course

When planning a zip course, designers look at topography, using hills, valleys and water features to their advantage. “People like to zip line because it makes them feel like they’re flying,” said Jon Johnson, owner and builder of Zipline Utah in Deer Creek State Park. “So, if you think about a bird flying through the air, you don’t want to fly over a parking lot. That’s no fun. You want to find some piece of ground that has unique features and aspects to it.”

In Ohio, Larry Gerstner, co-owner of Hocking Hills Canopy Tours, had some advantages when he designed their second zip course. “I’m a civil engineer by trade,” he said. “When we had our first course built, it was built by Bonsai, which is maybe the leading [zip course] builder in the country. I worked with them when they built it and picked up a lot of information to go with my engineering background.”

Since Gerstner already owned the property, he was familiar with its features when he designed the second course. “It’s more taking the terrain and the trees and using them to the best advantage. You need to start out high and end up low. We don’t have enough fall to do the whole thing. So all of our courses have a walk in the middle where you gain some elevation, so that you double-use the elevation.” Gerstner took advantage of the unusual Hocking Hills topography to end one zip line inside a shallow cave and cross a river several times.

cables attached to a tree trunk

Attaching cables to trees

The biggest danger to trees comes from attaching cables and bolts to them in order to hold up platforms. According to a report by the engineering firm Robson Forensic, “Drilling any hole into a tree creates a wound that makes the tree more susceptible to infection and decay.” So trees need to be chosen carefully. They should be healthy, sizable and of a species that can compartmentalize damage and limit the spread of decay around a wound.

“We need to protect them as much as we can, and we do love our trees,” Gerstner said. Hocking Hills Canopy Tours hired an arborist to evaluate its trees and pick the best candidates for platforms. Gerstner explained that when attaching platforms to trees, they stack bolts on top of each other so that the cambrium, or growth tissue of the tree, is only disturbed in one spot. “You’re only taking a little bit of the tree that you’re using out of commission for a brief period. It does grow back around that.” Other attachments to the tree use oak blocks to minimize damage. “We cut oak blocks and put them around the tree, and put our cable through the oak blocks so that it never touches the tree and the tree can grow.” Putting a cable all the way around a tree would kill it.

“We also ‘guy’ the trees, so that where we have the force of a zip line on the tree, we’ll have a guy that goes the opposite direction that holds that force, so there’s nothing really pulling on the tree,” Gerstner said. “The tree has to be able to move back and forth some, but we limit that moving back and forth to keep it strong.”

person walking along a sky bridge

Ongoing tree health

Once a course is in use, staff and arborists frequently inspect trees to ensure health. Adventures on the Gorge in West Virginia operates Tree Tops Canopy Tours, a 10-zip course with five sky bridges that wind through an ancient hemlock forest. According to Roger Wilson, CEO, its Bonsai-constructed course uses the block-and-cable system. “As trees grow, the blocks and cables can be readjusted with minimal impact on these ancient trees.”

In Trinidad, ZIP-ITT Adventure Tours operates a course including an 800-foot line with a stunning view of Macqueripe Bay. “We would like to think that we have enhanced many of the trees in our area as we continue to look after them, including regular spraying for termites,” said Matthew Devaux, ZIP-ITT’s director.

Gerstner watches his Ohio forest closely for sick or dead trees. “So it’s not quite a natural thing. We take better care of it. It’s a managed forest.”

In Utah, the way local wildlife interacts with zip lines has surprised Johnson. Unlike canopy courses that are built in trees, Johnson’s platforms are all on constructed towers. “We get a lot of hawks and a lot of bald eagles,” Johnson said. “We’ll see them swoop down and catch a fish in the lake. We’ll find half-eaten fish on our towers. They use our towers as perches.”

person hanging from a zip ling

Getting in touch with forests

Zip lines are more than just an adventure. They can be a powerful way for people to connect with forests. “We would like to think we have brought an awareness to the green environment and that feeling of being one with nature as you zip around our course,” Devaux said. “Moving into Macqueripe 6.5 years ago, we brought such an awareness to our staff, who now understand the effects of littering and the impact it has on the environment. They are now like ambassadors for the area, which is a great achievement for ZIP-ITT. The team has embraced keeping the forest clean and educating those who come about the area and the flora and fauna we enjoy because of it.”

Many zip guides make sure to slip some education into the adventures. Gerstner said that most of the Hocking Hills guides are ecology majors who talk about tree characteristics during the tour. “We get a lot of people from the city, and they’ve never been in the forest,” Gerstner said. “They get a bird’s eye view of being up in the trees. Most people never get that in their life.”

Images via Teresa Bergen / Inhabitat